1. Technical Field
This invention relates generally to monobloc pistons for diesel engine applications in which the piston skirt is formed as one integral piece with the piston body, and more particularly to the construction of the piston skirt.
2. Related Art
Monobloc pistons for diesel engine applications are known wherein the piston skirt is formed as an integral portion of the piston body, as opposed as being articulated, Such pistons are known to have piston skirts that are of such length and/or are positioned such that the lower edge of the skirt extends well below the bottom of the pin bosses to provide the desired support and guidance to the piston as it reciprocates within the cylinder of the engine. Such low lying skirts, however, can extend outside of the cylinder liner at bottom dead center position of the piston and can lead to scuffing of the piston skirt and/or liner due to the sudden change in load and direction as the piston returns upwardly in the cylinder. In addition, the low lying skirt interferes with the location of oil injection nozzles which direct cooling oil up into the piston, requiring the skirt to be notched in the area of the nozzle to provide clearance. Examples of such monobloc pistons are disclosed in U.S. Pat. Nos., 4,161,165 and 4,286,505, as well as published international application WO 9620340. The piston disclosed in the latter publication has a short, low lying piston skirt which is uncoupled from the ring belt and, due to the wide gap between the bottom of the ring belt and the top of the skirt as well as the low, remote positioning of the skirt, transfers some of the piston guidance duties to the ring lands near the top of the piston, which is less efficient and could cause damage to the ring lands as well as decrease the performance of the piston.
U.S. Pat. No. 4,704,950 discloses a high performance automotive piston for gasoline engines which is manufactured entirely of aluminum and would be unsuitable for high compression diesel applications to which the present invention is directed.
It is an object of the present invention to provide a monobloc piston for diesel engines that overcomes or greatly minimizes the deficiencies of the prior art pistons described above.
A piston for diesel engines constructed according to a presently preferred embodiment of the invention comprises a piston body fabricated of steel having an outer wall and a closed oil gallery. A pair of pin bosses have axially aligned pin bores and a lowest tangency point of the pin bores. The piston body has a skirt that is formed as one piece with the pin bosses and is coupled at its upper end to the outer wall so as to form a continuous extension of the outer wall. The piston skirt has a lower edge spaced below the pin bore axis, but located at or above the lowest tangency point of the pin bores.
The invention also contemplates a diesel engine which includes an engine block having at least one cylinder bore and a monobloc piston disposed in the cylinder bore having a piston body fabricated of steel and including a pair of pin bosses with aligned pin bores disposed about a pin bore axis and having a lowest tangent point to the pin bores. A piston skirt is formed as one piece with the pin bosses and has a lower edge disposed below the pin bore axis at or above the lowest tangent point of the pin bores.
The invention has the advantage of providing a steel monobloc piston for diesel engine applications fitted with a short skirt made of the same steel material of such size and location relative to the remainder of the piston body to provide efficient guidance to the piston during reciprocation in the cylinder bore to reduce loading on the ring lands due to its high location relative to the pin bores. The high location of the lower edge of the skirt prevents the skirt from extending from the bottom of the cylinder bore at bottom dead center of the piston, and thus minimizes or eliminates scuffing of the skirt caused by a sudden change in load or direction when the piston returns upwardly. The relatively high location of the lower edge of the skirt also provides ample clearance for the oil jet nozzles, eliminating the need for clearance notches in the lower edge of the skirt so as to present a continuous, non-interrupted lower leading edge of the skirt which is stronger and easier to manufacture than notched skirts.
The overall reduction and the height of the skirt further has the advantage of simplifying the manufacture of forged steel pistons. The shorter length enables production of a relatively thinner, more uniform thickness skirt wall as compared to forged skirts of greater length which are generally thicker due to the required draft angle to enable forging of the piston skirt.